If you plan to travel around the holidays, prepare to pay a little more — again.

Most of the largest U.S. airlines have increased a surcharge for travel on the busiest travel days to $20 each way, up from $10.

The surcharges apply to a large number of flights within the U.S. on more than a dozen peak days around Thanksgiving, Christmas and New Year's.

Delta, American, United, US Airways and Northwest all boosted the surcharge on some routes, said Tom Parsons, who runs the discount travel site Bestfares.com.

Tim Smith, spokesman for Fort Worth-based American Airlines, confirmed the higher surcharges Monday. He said that although airlines are filing the increases as a surcharge this time, "fares on those peak days have always tended to be higher. It's a matter of supply and demand."

Smith said the increases started late last week with US Airways, and "most other airlines, including American, have matched."

US Airways spokesman Morgan Durrant confirmed the higher peak day surcharges, but he said his airline did so only "to match moves by our competitors."

Delta, its Northwest subsidiary and United also raised the surcharges to $20 each way on many U.S. routes, according to representatives.

Parsons, the travel website operator, said the increases were part of a clear trend in airline pricing.

"With airlines downsizing, fuel going up and airlines still losing money," he said, "we're going to pay more for family vacations going into 2010."